This invention relates generally to video systems for the storage and retrieval of documentary information, and more particularly to a video system including a multi-head magnetic tape recorder for recording and playing back video images while the magnetic tape is stationary.
Video filing systems are known (see U.S. Pat. Nos. 3,594,729 and 3,514,537) which are adapted to record and store documentary information whereby a large body of information may be concentrated in a compact bank from which it can readily be retrieved on demand. In one such video filing system, paper documents are converted by a high-resolution video camera into corresponding high-resolution video image signals. These signals, together with identifying addresses, are automatically filed and stored on magnetic tape reels.
In this video filing system, any individual document page can automatically be retrieved, looked at in its original size, purged, reorganized with other images or shifted to various locations. An image of a recalled document is presented for viewing on a high-resolution television screen from which it can be reproduced as a hard copy. Since the document images are electronic in nature, filing and retrieval can be carried out remotely from a central file.
The crucial cost factor in a video filing system of this type lies in its means to store the documents as compact video images on magnetic tape which, when played back, are as readable as the original documents. The fact that commercial television systems afford clear images does not means that such systems are suitable for document storage and retrieval. There is a vast difference between being able to read on a T-V screen an 81/2 .times. 11 inch document having more than a thousand characters printed on the page, and being able to see on the screen a picture of a house, for in the latter instance, gross detail is sufficient to give one a clear impression of a house, whereas in the former, small printed characters cannot be deciphered.
Thus a video filing system must employ high-resolution video means for recording and displaying documents. An electronic image of a document is created by scanning an optical image of the document focused on the photo-sensitive surface of the video camera tube. Scanning is effected by sweeping an electron beam across the sensitive surface, each sweep being a scan line. By the time the beam has sequentially scanned across the entire picture area from top to bottom, it has created an electronic image of the original document to complete an image frame.
The number of scan lines in an image frame determines one dimension of its resolution or readability. Resolution is a measure of how readable a document is when retrieved from the video picture. Commercial television in the United States has an established standard of 525 scan lines per frame. The resultant resolution is altogether inadequate for normal printed matter. Hence in one known video filing system, use is made of a high-resolution camera and a display tube having 1,280 scan lines in each frame. The other dimension involved in resolution is normally determined by the upper frequency limit of the system.
With a high-resolution system of the known type, the magnetic tape storage components and all other functioning elements of the system are designed to operate with the high-resolution scan line number. As a consequence of this requirement, use cannot be made of commercially-available video recording components designed to operate with the standard 525 scan line number per frame.
Another drawback of the known system which discourages its adoption is that when video signals from the storage bank are to be transmitted over common video carrier lines to a remote user terminal, one cannot use standard T-V transmission facilities for this purpose, for such facilities are incapable of conveying the frequencies of a high-resolution video signal. For example, even if a common video carrier line is capable of carrying a 7 megacycle high-resolution video signal as well as the standard 4 megacycle video signal, the associated synchronization system which is designed for the existing standard will not function with the high resolution signal. Hence, special carrier lines are called for, and this fact adds considerably to installation and operating costs.
The practical consequences of these restrictions are serious and have discouraged the adoption of video filing systems; for while standard components are mass-produced, high-resolution recording equipment is not an off-the-shelf item. High resolution devices must be custom-manufactured and inevitably are far more expensive than standard equipment.
With a view to overcoming the practical limitations of a video filing system of the above-described type, the Goldberger U.S. Pat. No. 3,803,352 discloses a hybrid high-resolution/low-resolution video information storage and retrieval system. The entire disclosure of this patent is incorporated herein by reference. In the Goldberger system, the video camera tube for converting the documents into video signals and the video display tube on whose screen the stored documents are reproduced both function as high-resolution devices with a scan line number per frame that is a predetermined multiple of the standard T-V low resolution number and with a frame repetition rate that is a complementary sub-multiple of the standard rate.
For example, in the Goldberger system, a preferred high-resolution frame line number is 1,575, which is exactly three times the standard 525-line number, in which event the sub-multiple is 10 frames per second, which is exactly one-third the standard frame repetition rate of 30 frames per second. Thus if the standard scan line number is multiplied by three, the frame rate is divided by the same factor.
As pointed out in the Goldberger patent, the multiple/sub-multiple relationship between the standard T-V values and the selected high-resolution T-V values need not be 3:1/3 but may be higher, such as 4:1/4 or 5:1/5. For purposes of illustrating the present invention, a 2:1/2 relationship has been chosen, but it will be apparent that other relationships are feasible.
The video signals generated by the high-resolution camera in the Goldberger system are stored in a standard low-resolution magnetic storage device. In order to reconcile this low-resolution apparatus with the high-resolution camera and with a high-resolution display tube, a buffer is provided that functions to divide the video signals representing a single image frame into distinct signal fields, each having a scan line number equal to the standard line number per frame within a time period equal to the full frame period of the standard frame repetition rate.
For this purpose, Goldberger makes use of a buffer to temporarily record and store a single frame, the buffer being in the form of a disc-type recorder whose operation is controlled by a switching circuit. The buffer is adapted to accept the 1,575 line per frame video signal (10 frames per second) from the camera and to divide this signal equally among three parallel continuous tracks on the disc recorder, whereby the first 525 lines of the full frame, which appear during a one-thirtieth of a second interval, go to the first continuous track, the second 525 lines, which appear in the next one-thirtieth of a second, go to the second continuous track, and the final 525 lines of the same frame, which appear in the last one-thirtieth of a second, go to the third continuous track. Thus the high-resolution video signal is divided into three equal signal fields, each of which has the standard scan line number and frame repetition ratio. However, each field represents only one-third of the total high-resolution frame.
In playback, the buffer disc is continuously rotated to repeat the image frames to provide a stationary image on the T-V screen for as long as the user requires the image. With a low-resolution disc recorder in which the course of a full disc revolution takes place in one-thirtieth of a second, a recording of a single high-resolution frame from the video camera takes place sequentially on three tracks in the course of three revolutions. Thus, a full high-resolution frame composed of three signal fields is recorded in one-tenth of a second.
If one were to play back any one continuous track on the disc on a standard T-V display tube, one would see only a third of the original document, which would appear at the standard 525 line-30 frame per second rate. In order, therefore, to reconstruct the document, all three tracks must be played back in sequence.
The buffer disc recorder is provided with a unitary assembly of three recording heads, each associated with one of the three tracks in a disc having a large number of concentric tracks. The switching circuit, which is controlled by the T-V camera, acts to render the first head operative for the first signal section of 525 lines, the second head being rendered operative for the next 525 lines and the third head for the final 525 lines.
Thus the three continuous tracks on the buffer disc are recorded in sequence. When another document is to be recorded, the tri-head assembly is automatically mechanically indexed to the next set of three tracks on the disc. Since the buffer disc serves only for temporary storage, means are provided to erase the recordings after the buffer has performed its required function. Since a document frame appears in a set of three continuous tracks on the buffer disc, it may be transferred to the magnetic tape recorder which is in the standard format.
Hence in the Goldberger system, in order to store the high-resolution video signals produced by the high-resolution camera, the signals must be recorded temporarily on the separate track of a rotating buffer disc and then transferred from the buffer disc to magnetic tape for permanent storage therein, whereas in the playback mode, the fields recorded on the magnetic tape must be returned to the disc and from there fed to the high-resolution display tube.